Coding of signals for transmission over a digital channel generally includes sampling an input, quantizing the samples, and forming a digital code for each quantized sample. A replica of the input is produced by decoding the digital codes corresponding to the input into a time sequence of signal samples and low pass filtering the sample sequence. As is well known in the art, some types of signals such as speech patterns are highly correlated so that portions thereof can be predicted from past values. By taking advantage of the predictive nature of such signals, the channel bit rate may be substantially reduced.
Predictive coding of signals as disclosed in U.S. Pat. No. 3,631,520, issued to B. S. Atal, Dec. 28, 1971, and assigned to the same assignee, involves the generation of predictive parameters from a succession of signal samples and the formation of a predicted value for each sample from the generated parameters and the preceding signal samples. The difference between each sample and its predicted value is quantized, encoded and sent to a receiver wherein the difference signal is decoded and combined with the corresponding predictive value formed in the receiver.
U.S. Pat. No. 4,411,003, issued to James C. Su, Oct. 18, 1983, discloses a multiloop adaptive Delta Modulator in which a delta modulated bit stream is applied to a filter having selectable filter characteristics dependent on the autocorrelation functions. In this way the bit stream may be modified in accordance with the type of speech pattern being coded. While this patent demonstrates that signal improvement may be achieved by considering the characteristics of the speech pattern applied to coding apparatus, it does not disclose the utilization of the adverse effects of quantization as a means of adapting the coding process to improve speech signal quality.
Quantizing a signal sample is accomplished as is well known by selecting the closest of a set of specified amplitude levels. The approximation introduced by quantization, however, results in noiselike distortion. Such quantization noise may be reduced by forming an error signal corresponding to the difference between the quantized and unquantized signals and modifying the signal samples in a prescribed manner responsive to the error signal. While the total quantizing noise level is unaffected by the modification, the noise may be reshaped so that it is concentrated in a specific portion of the signal spectrum where its effects are minimized. For speech signals, quantizing noise may be concentrated in formant regions of the speech signal spectrum. This results in the noise being masked so that it is not perceived by the listener.
U.S. Pat. No. 4,133,976 issued to B. S. Atal et al, Jan. 9, 1979, and assigned to the same assignee discloses a circuit adapted to redistribute quantizing noise in a speech signal spectrum during the digital encoding of the signal. The use of such a circuit in a digital coding scheme results in a significant reduction in the perceived quantizing noise. The circuit arrangements, however, are relatively complex and presume that the communication system operates at a predetermined bit rate.
The article "Enhancement of ADPCM Speech by Adaptive Postfiltering" by V. Ramamoorthy and N. S. Jayant appearing in the AT&T Bell Laboratories Technical Journal, pp. 1465-1475, October 1984, discloses an arrangement to reduce quantizing noise effects in a variable bit rate communication system in which the transmission bit rate at a predictive decoder is detected and the predictively decoded signal is modified responsive to the bit rate and the prediction parameters. The arrangement requires use of bit rate signals in the filtering circuitry and assumes the noise level of the predictively decoded signal varies only as a function of bit rate. It is known, however, that the noise level varies widely at each bit rate. While it is generally known that the quality of digital speech transmission is poorer at lower bit rates, filtering as a function of transmission bit rate as in the V. Ramamoorthy and N. S. Jayant article does not account for variations in quality of the coded speech signal at a particular bit rate. It is an object of the invention to provide improved digital signal coding that reduces quantizing noise effects for communication at a variable bit rate as a function of the noise in the predictively decoded signal.